Fiber lasers and fiber amplifiers can now be operated at high power levels ranging from hundreds of watts to kilowatts of optical power. Such laser sources have many industrial applications, such as marking, cutting and welding of various materials. High power fiber lasers and amplifiers are all-silica structures making them robust against environmental factors. They also require only minimal service, in contrast to other laser types used in industry.
An optical resonator cavity of a fiber laser is generally built from components, such as gain fiber, fiber Bragg gratings, and pump couplers that are spliced together to form the cavity. While the cavity construction is generally all-glass, meaning that the laser signal radiation inside the resonator is confined inside silica glass at all times, there are situations where some of the radiation leaks out from a glass component into free space. As an example, some constructions of a pump coupler are such that part of the radiation that gets introduced into the coupler in reverse direction, i.e. direction opposite to that of pump radiation, leaks out from the glass-construction of the component into free-space air. The reverse radiation may be introduced e.g. by back-reflection from the materials processing target, or from radiation that is coupled into the cladding of the gain fiber. The power of radiation introduced into free-space within the package of a component may easily be tens to hundreds of watts in a high-power fiber laser operating at kW power levels.
Exemplary coupler and structures are disclosed e.g. in US 2010/0142894, U.S. Pat. No. 7,492,993 and U.S. Pat. No. 6,546,169.
The free-space radiation introduced inside the package of a component is a potential risk for the component. To be more specific, the all-glass component is generally fixed to a housing in order to protect the component from mechanical or environmental factors. Fixing of the component is generally done with a glue or some other polymer material. When the free-space radiation hits the polymer, it gets at least partially absorbed there. The polymer has relatively poor thermal conductivity, and gets therefore heated up by the absorbed radiation. Heating may be so severe that the temperature of the polymer exceeds the failure temperature of the polymer. This situation will lead to runaway process of temperature, charring of the polymer, and eventually melting of the glass of the component. In other words, the component is destroyed, and the fiber laser the component is part of also fails.
An exemplary fiber-optic coupler structure is shown in FIG. 1a, which represents prior art. It comprises a plurality of input fibers 11 (two of which are shown), a coupling structure 12, and an output fiber 13. Optical radiation in the reverse direction (arrow 18′) is incident to the coupler from the output fiber. This radiation may be due to unabsorbed pump radiation from the other side of the cavity, cladding radiation originated at fiber splices or other imperfections, or back-reflections from the target material when the fiber laser is used in real-world applications. A large fraction of this radiation may propagate in the glass cladding of the output fiber. The reverse propagating radiation spreads inside the glass structure of the coupling structure 12 (arrows 18″). When the radiation reaches the end surface of the coupling structure, part of it radiates into free space between and outside of the input fibers, as shown in FIG. 1a (arrows 18′″). This is due to the fact that while the component is generally designed to operate with high transmission of radiation in the forward direction, the transmission in reverse direction is usually not nearly as good. Since the reverse propagating radiation may be of low numerical aperture or low divergence, the portion of it radiating into free space may form an almost collimated beam of light.
FIG. 1b shows a packaged fiber-optic coupler. The package 15 is typically made of metal, and it has a groove or slit into which the component is mounted. The coupler is fixed to the housing from both ends with polymer zones 16A, 16B which is optical epoxy or alike glue material. Free-space reverse radiation 18 from the coupling structure is propagating towards the glue on the input fiber end of the package, and part of the free-space reverse radiation 18 hits the glue, and gets absorbed by it. It should be noted that even though clear optical epoxies are used as a glue, their absorption is still sufficiently high for the absorbed radiation to be able to heat the glue to high temperatures. This is particularly true for couplers operating in high power fiber laser systems, where the power of the free-space radiation may be several tens of Watts. Often the free-space radiation is also pretty collimated and may thus produce power densities in the range of kW/cm2. Therefore, even if the glue is heat sunk to the housing, the glue may be severely heated by the radiation. The failure temperatures of typical optical glues are around 100° C., above which the thermal runaway can happen. The temperature of failure may be defined by the glass transition temperature of the polymer, or by temperature dependent absorption characteristics of the material. The failure may be visible as the collapsing of physical strength of the polymer or by thermal runaway and eventual charring of the material. Both processes usually lead to the failure of the fragile glass component.
US 2003/0103753 discloses a packaged optical coupling device which contains a quartz substrate suspended on lateral sides of the packaged component and designed to transport leaked light energy away from the packaged component and towards the housing of the component, where it is absorbed. Thus, this solution is essentially based on increasing the gap between the component and its housing using transparent glass in order to reduce the local intensity of the light absorbed to the housing. This solution has the disadvantage of increasing the dimensions of the package and not protecting the device from reverse radiation leaked parallel to the fibers of the device. In particular the area, where the fibers are mounted to the package, is not protected from reverse radiation.